Semiconductor light sources, systems, and methods

ABSTRACT

A light-emitting diode includes a substrate, a lower cladding layer, an active layer having a quantum well of a thirty percent concentration of indium on the lower cladding layer, and an upper cladding layer. A method of manufacturing light-emitting diodes includes forming a lower cladding layer on a substrate, forming an active layer on the lower cladding layer such that the active layer has a quantum well of thirty percent indium, forming an upper cladding layer on the active layer, and forming a metal cap on the upper cladding layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of co-pending U.S. patent application Ser. No.12/028,516, filed Feb. 8, 2008, entitled SEMICONDUCTOR LIGHT SOURCES,SYSTEMS, AND METHODS, which is a non-provisional of U.S. ProvisionalPatent Application No. 60/888,874, filed Feb. 8, 2007, entitledSEMICONDUCTOR LIGHT SOURCES, SYSTEMS, AND METHODS, the disclosure ofwhich is herein incorporated by reference in its entirety.

BACKGROUND

The disclosure generally relates to light emitting semiconductor devicesand systems. It particularly relates to systems used for IR inspectionand imaging of single and bonded semiconductor substrates, MEMS(MicroElectro-Mechanical Systems) and MEMS-type constructions andbio-sensors.

A semiconductor light source, such as a light emitting diode, sourcewith a peak wavelength operating in the 1.0 to 1.1 μm band is veryuseful for certain known and future applications. For example, thewavelength may be used for MEMs inspection or the ability to see throughsemiconductor wafers. The wavelength may be detected by certaindetection means, such as standard hi-resolution CCD (charge-coupleddevice) arrays. The wavelength also has medical applications, forexample, blood analysis of diabetics.

Current illuminators and literature in this area refer to laser diodesources for these applications. These are not ideal for this type ofapplication as laser speckle, scatter and lack of uniformity causedifficulties for inspection type applications.

Current implementations use a strained InGaAs quantum well surrounded byGaAs cladding layers and have been well documented in the peer reviewedjournals and in some patents. All references focus on the use of thesestructures for laser diode and not other semiconductor light sources,such as light emitting diode (LEDs) type illuminators, and certainly notin arrays of LEDs. Manufacture of arrays of laser diodes is notpractical, making LEDs a more attractive implementation in which toachieve the desired wavelengths. However, as mentioned above attainingthe desired wavelengths in a structure that can be manufactured inarrays creates some difficulties.

Another difficulty with the current designs arises in trying to extendthe wavelength of the peak emission wavelength beyond 1 μm. To achievethis, the indium content of the InGaAs strained quantum well has to beincreased and the quantum well thickness also increased. The thicknessapproaches the critical thickness for the quantum well. Thicknesseslarger than the critical thickness causes defects in the layers, in turnleading to a relaxation of the lattice that may result in device failureand poor performance.

SUMMARY

One embodiment is a light-emitting diode includes a substrate, a lowercladding layer, an active layer having a quantum well of a thirtypercent concentration of indium on the lower cladding layer, and anupper cladding layer. Another embodiment is a method of manufacturinglight-emitting diodes includes forming a lower cladding layer on asubstrate, forming an active layer on the lower cladding layer such thatthe active layer has a quantum well of thirty percent indium, forming anupper cladding layer on the active layer, and forming a metal cap on theupper cladding layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of a LED layered structure.

FIG. 2 shows an embodiment of a LED layered structure having a Braggreflector.

FIG. 3 shows an embodiment of a LED layered structure having straincompensation.

FIG. 4 shows an embodiment of a process for fabricated an LED structure.

FIG. 5 shows an embodiment of an LED from a side view.

FIG. 6 shows an embodiment of an LED array from a top view.

DETAILED DESCRIPTION

The foregoing is not intended to be an exhaustive list of embodimentsand features of the present inventive concept. Persons skilled in theart are capable of appreciating other embodiments and features from thefollowing detailed description in conjunction with the drawings.

New LEDs may be produced from processes using a III-V compound substrateon which is grown a number of layers by a process such as molecular beamepitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). Bychanging the composition of the III-V compound semiconductor layers andthe typical layer structure one can produce arrays of LEDs that canattain a 1.0 to 1.1 μm bandwidth.

The LED is produced from a III-V compound substrate on which is grown anumber of layers by a process such as molecular beam epitaxy (MBE) ormetal-organic chemical vapor deposition (MOCVD). A “III-V” compoundrefers to a compound having an element from group III on the PeriodicTable of the Elements, a metallic element, and an element from group Von the Periodic Table, a nonmetallic element. One common examplecompound used in manufacture of LEDs is gallium (Ga) arsenide (As).

A layered LED structure is shown in FIG. 1. It must be noted that thesestructures have several layers, numbered consecutively. No suchlimitation is intended nor should it be implied. These are exemplarystructures and are merely used as means to promote understanding of theembodiments.

In one embodiment, the LED layered structure has a quantum well in theactive layer, with no strain compensation. The compositions of thelayers are shown in the table below, with layer 5 being the activelayer.

Com- Thick- Layer position ness Doping Dop- Number Material Type (x)(μm) Level Type ant 9 GaAs 0.1 1 × 10¹⁹ p ++ Zn 8 Al(x)Ga(1 − x)As 0.34.0 6 × 10¹⁸ p + Zn 7 Al(x)Ga(1 − x)As 0.0-0.3 0.2 3 × 10¹⁸ p + Zn grade6 GaAs 0.05 5 In(x)Ga(1 − x)As 0.3 90 {acute over (Å)} 4 GaAs 0.05 3Al(x)Ga(1 − x)As 0.7-0.0 0.2 3 × 10¹⁸ n + Si grade 2 Al(x)Ga(1 − x)As0.7 1.8 3 × 10¹⁸ n + Si 1 GaAs 0.2 3 × 10¹⁸ n + Si

In the above embodiment, the strained quantum well is designed to havepeak photoluminescence (PL) at 1070 nm±5 nm. The gallium arsenidesubstrate is 2° off (100) n+Si 1−4×10¹⁸. Combinations of the layers oneither side of the active layer, such as layers 2-4 and 7-9, eitheralone or in other combination, may be referred to as upper and lowercladding layers. The active layers, such as layers 4-6, alone or incombination may be referred to as the active layer, event though theactive layer may comprise several layers.

If a standard LED layer is used, the level of strain in the quantum wellis such that it approaches the critical layer thickness for the quantumwell and this will cause difficulty in the reliability of this device.Adjusting the layer thickness of the quantum well and the amount of theindium concentration in the well allows tailoring of the peak emissionwavelength in the 0.98 to 1.07 μm as required. However, for wavelengthsapproaching 1.1 μm difficulties in the critical layer thickness andindium concentration lead to problems in achieving the desired peakwavelength.

As an added feature of this embodiment, the bottom cladding layer, layer2 in the above table, has a higher concentration of aluminum than thetop cladding layer, layer 8 in the above table. This reduces therefractive index and acts as guide reflecting some emitted light backinto the active, out of the sides of the chip and out of the top of LEDdie. All of this light contributes to the total light output from thelight emitting diode, increasing the efficiency of the LED, which isgenerally the intensity of the light or number of photons produced for agiven applied current, or number of electrons introduced, to the p-njunction forming the diode.

In another embodiment, the layers are manufactured as above, with theaddition of a Bragg reflector, or Bragg stack, into the layers.

Com- Thick- Layer position ness Doping Dop- Number Material Type (x)(μm) Level Type ant 11 GaAs 0.1 1 × 10¹⁹ p ++ Zn 10 Al(x)Ga(1 − x)As 0.34.0 6 × 10¹⁸ p + Zn 9 Al(x)Ga(1 − x)As 0.0-0.3 0.2 3 × 10¹⁸ p + Zn grade8 GaAs 0.05 7 In(x)Ga(1 − x)As 0.3 90 {acute over (Å)} 6 GaAs 0.05 5Al(x)Ga(1 − x)As 0.7-0.0 0.2 3 × 10¹⁸ n + Si grade 4 In(x)Ga(1 − x)As 0.11 0.075 3 × 10¹⁸ n + Si 3 In(x)Al(1 − x)As 0.1 0.089 3 × 10¹⁸ n + Si2 Al(x)Ga(1 − x)As 0.7 1.8 3 × 10¹⁸ n + Si 1 GaAs 0.2 3 × 10¹⁸ n + Si

In the above embodiment, layers 3 and 4 repeat. In a specific example,layer 3 may repeat 25 times and layer 4 may repeat 24 times, forming theBragg stack. This is shown in FIG. 2.

The LED generates infra-red illumination in the active layer but mayemit light along a path normal, or close to normal, to the perpendicularof the chip. The substrate or bottom contact typically absorbs thisillumination. The addition of a Bragg stack will act as a reflector forthese photons and a typical photon will then be emitted from the topsurface of the die.

The addition of this Bragg stack used in the manufacture of surfaceemitting laser diodes will improve the quantum efficiency for LEDs. Theefficiency is generally the amount of photons emitted by the LED chipdivided by the number of electrons injected into the active region, byapproximately 30%. This has benefits in the amount of light generated bythe chip and reduces the amount of excess heat generated improving thereliability and performance of these devices.

In the above embodiments, the quantum well did not have any straincompensation. Strain occurs in the quantum wells due to the thickness ofthe layers and their compressive stress. In strain compensation, layerswith tensile stress are introduced on either side of the compressivelystrained quantum well layer. This compensates for the compressive stressand increases the barrier heights. An example structure is shown in FIG.3, with the layers being comprised as shown in the below table.

Com- Thick- Layer position ness Doping Dop- Number Material Type (x)(μm) Level Type ant 11 GaAs 3.0 1 × 10¹⁹ p ++ Zn 10 Al(x)Ga(1 − x)As 0.33.0 2 × 10¹⁸ p + Zn 9 Al(x)Ga(1 − x)As 0.0-0.3 0.2 2 × 10¹⁸ p + Zn grade8 GaAs(1 − x)P(x)  0.07 240 {acute over (Å)} 7 In(x)Ga(1 − x)As 0.3  65{acute over (Å)} 6 GaAs(1 − x)P(x)  0.07 240 {acute over (Å)} 5In(x)Ga(1 − x)As 0.3  65 {acute over (Å)} 4 GaAs(1 − x)P(x)  0.07 240{acute over (Å)} 3 Al(x)Ga(1 − x)As 0.7-0.0 0.2 3 × 10¹⁸ n + Si grade 2Al(x)Ga(1 − x)As 0.7 1.8 3 × 10¹⁸ n + Si 1 GaAs 0.2 3 × 10¹⁸ n + Si

In this embodiment, barrier layers of GaAsP are introduced. The layersprovide a partial compensation for the compressively strained quantumwell and increase the barrier heights. Phosphorous is used in thisexample, but other elements may be used in its place. This eliminatesany issues with the thickness of the quantum well, as the straincompensation at least partially neutralizes the stresses that may causedefects in the layers that result in device failure.

The above structure may or may not include the Bragg stack discussedabove. No limitation to these particular layers is intended, nor shouldit be implied.

Adjusting the thickness and percentage of Indium in the quantum well andmaking a corresponding compensating change to the GaAsP barriers, it ispossible to adjust the peak wavelength of the Light Emitting Diodeanywhere in the 1.0 to 1.1 μm band.

Using these layers, normally used in laser diodes, it is possible tomanufacture LED arrays that produce wavelengths in the desired bands. Aprocess for manufacture is shown in FIG. 4.

The LED structure is formed at 20, for example, as discussed above. Forexample, the structure is formed with a higher than normal concentrationof Indium in the quantum well, upwards of 30%. A Bragg reflector may ormay not be included with that structure. Alternatively, the device mayinclude strain compensation in the form of cladding layers havingtensile stress.

A top contact is formed at 22. The top contact is generally a ‘cap’ ofmetal, such as titanium-platinum-gold, (TiPtAu), is formed over each LEDelement. This is shown in FIG. 5, with the GaAs top layer with nometallization at 34, and the cap shown as 32. Generally, the top contactwill be formed using a lift-off process, rather than an etch process,due to the multiple layers of metal and gold contact. The use of aTiPtAu cap produces an ohmic, p+ cap.

At 24, the LED carrier upon which the LED structures were formed isprepared for dicing. This may involve thinning the back of the substrateupon which the LED is formed. A back metal, such asgold-germanium-nickel-gold (AuGeNiAu) layer may be deposited, shown at36 in FIG. 5, and annealed.

The LED carrier is then diced at 26 and the LEDs are tested at 28.Testing may involve voltage testing all of the device and sampling thepower and peak wavelengths. All of the devices that pass the testingphase would then be mounted.

Mounting, at 30 in FIG. 4, may involve mounting the die, or multipledie, on blue film. At 32, electrical contacts would be formed betweenthe base of the die using solder or electrically conductive adhesive. Acontact is made to the cap using gold or aluminum wire.

In operation, when electrical current is passed through the LED, the LEDemits light at the desired spectral wavelength. An LED having theability to emit light with wavelengths in the 1.0 to 1.1 μm range is newand may be used in many different applications, including semiconductorand MEMS device inspection.

One such application is in an array of LEDs, as shown in FIG. 6. The topof each LED is shown with its metalized cap 32 formed into an array 40.The use of the different layer compositions discussed above make itpossible to manufacture arrays of quantum well LEDs, rather than singlelaser diodes, to produce light in the desired wavelength.

Exemplary solid-state light sources, systems and applications in whichthe die contemplated herein may be used in include those set forth inPCT Patent Application PCT/US03/14625, filed May 8, 2003, entitled “HighEfficiency Solid-State Light Source and Methods of Use and Manufacture,”hereby incorporated by reference in its entirety for all its teachings.PCT Patent Application PCT/US03/14625 discloses, among other things,high-intensity light sources that are formed by a micro array ofsemiconductor light sources, such as LEDs, laser diodes, or VCSEL placeddensely on a substrate to achieve power density output of at least 50mW/cm².

Exemplary solid-state light sources, systems and applications in whichthe die contemplated herein may be used in include those include thosedisclosed by U.S. patent application Ser. No. 11/109,903, filed Apr. 19,2005, entitled “Imaging Semiconductor Structures Using Solid StateIllumination,” which is hereby incorporated by reference. U.S. patentapplication Ser. No. 11/109,903 discloses, among other things a solidstate light source that irradiates selected semiconductor structures viaa fiber optic light guide and a lens system. The source's radiation isdirected to structures via an internal beam splitter in the lens system.The radiation, so directed, generally is reflected by structures atvarious intensities (e.g., depending on the bond characteristics andother features and defects of the semiconductor structures), so as totravel back up through the lens system, to a camera , such camera beingbased on or using one or more solid state imaging devices, e.g., CCD orCMOS detectors.

The camera preferably detects such reflected radiation of one or morewavelengths. Via such detection, an image of the structures is captured.The image, so captured, may be provided for further processing via,e.g., computer The captured image, so processed or otherwise, may beemployed for test and quality control, toward identifying relevantfeatures of such structures e.g., where such relevant features areassociated with bonded or stacked layers (e.g., in the interfacinglayer(s) of bonded or stacked substrates or in the bond itself) or withother bonded or stacked materials.

U.S. Pat. No. 6,995,405, by Braddell, et al, granted Feb. 7, 2006,entitled “Illuminator”, is hereby incorporated by reference.

Persons skilled in the art will recognize that many modifications andvariations are possible in the details, materials, and arrangements ofthe parts and actions which have been described and illustrated in orderto explain the nature of this inventive concept and that suchmodifications and variations do not depart from the spirit and scope ofthe teachings and claims contained therein.

1. A method of manufacturing light-emitting diodes, comprising: forminga lower cladding layer on a substrate; forming an active layer on thelower cladding layer such that the active layer has a quantum well ofthirty percent indium; forming an upper cladding layer on the activelayer; and forming a metal cap on the upper cladding layer.
 2. Themethod of claim 1, wherein forming at least one of the lower claddinglayer, the active layer, and the upper cladding layer using molecularbeam epitaxy.
 3. The method of claim 1, wherein forming at least one ofthe lower cladding layer, the active layer, and the upper cladding layerusing metal organic chemical vapor deposition.
 4. The method of claim 1,wherein forming a metal cap further comprising depositing a metal layeron the upper cladding layer and performing a lift off to leave aselective portion of the metal layer on the upper cladding.
 5. Themethod of claim 1, further comprising forming multiple light-emittingdiodes in an array.
 6. The method of claim 1, further comprising forminga Bragg stack between the lower cladding layer and the active layer. 7.The method of claim 1, further comprising forming a strain-compensatinglayer adjacent the active layer.